Steel sheet and method for producing the same

Abstract

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure including martensite and bainite. The total area fraction of the martensite and the bainite to the entirety of the microstructure is 95% or more and 100% or less. The balance of the microstructure is at least one of ferrite and retained austenite. The microstructure includes specific inclusion clusters, the content of the inclusion clusters in the microstructure being 5 clusters/mm2 or less. The microstructure includes prior-austenite grains having an average size of more than 5 μm. The steel sheet has a tensile strength of 1320 MPa or more.

The invention claimed is: 
     
       1. A steel sheet having a chemical composition comprising, by mass %:
 C: 0.13% or more and 0.40% or less, 
 Si: 1.5% or less, 
 Mn: 1.7% or less, 
 P: 0.030% or less, 
 S: less than 0.0010%, 
 sol. Al: 0.20% or less, 
 N: 0.0055% or less, 
 O: 0.0025% or less, 
 Nb: 0.002% or more and 0.035% or less, 
 Ti: 0.002% or more and 0.040% or less, and 
 the balance being Fe and inevitable impurities, 
 wherein the steel sheet has a microstructure including martensite and bainite, the total area fraction of the martensite and the bainite to the entirety of the microstructure being in a range of 95% or more and 100% or less, and the balance, if any, being at least one of ferrite and retained austenite, the microstructure including (i) prior-austenite grains having an average grain size of more than 5 μm , and (ii) inclusion clusters having a major axis of 20 to 80 μm, the content of the inclusion clusters in the microstructure being 5 clusters/mm2 or less, each of the inclusion clusters constituted by at least one inclusion particle, where:
 in the case where each of the inclusion clusters is constituted by one inclusion particle, the inclusion particle has a major axis of 20 to 80 μm, and 
 in the case where the inclusion clusters are constituted by two or more inclusion particles, the inclusion particles have a major axis of 0.3 μm or more, and the shortest distance between the inclusion particles is 10 μm or less, 
 
 Formula (1) and Formula (2) are both satisfied:
   [% Ti]+[% Nb]>0.007   (1)
 
   [% Ti]×[% Nb] 2 ≤7.5×10 −6    (2)
 
 where [%Nb] and [%Ti] represent the contents (%) of Nb and Ti, respectively, and 
 
 the steel sheet has a tensile strength of 1320 MPa or more. 
 
     
     
       2. The steel sheet according to  claim 1 , the chemical composition further comprising, by mass %, at least one group selected from Groups A-E:
 Group A: B: 0.0002% or more and less than 0.0035%, 
 Group B: at least one element selected from the group consisting of Cu: 0.005% or more and 1% or less, and Ni: 0.01% or more and 1% or less, 
 Group C: at least one element selected from the group consisting of:
 Cr: 0.01% or more and 1.0% or less. 
 Mo: 0.01% or more and less than 0.3%, 
 V: 0.003% or more and 0.45% or less, 
 Zr: 0.005% or more and 0.2% or less, and 
 W: 0.005% or more and 0.2% or less, 
 
 Group D: at least one element selected from the group consisting of:
 Ca: 0.0002% or more and 0.0030% or less, 
 Ce: 0.0002% or more and 0.0030% or less. 
 La: 0.0002% or more and 0.0030% or less, and 
 Mg: 0.0002% or more and 0.0030% or less, and 
 
 Group E: at least one element selected from the group consisting of Sb: 0.002% or more and 0.1% or less, and Sn: 0.002% or more and 0.1% or less. 
 
     
     
       3. The steel sheet according to  claim 2 , wherein the steel sheet has a coating layer disposed on a surface thereof. 
     
     
       4. The steel sheet according to  claim 1 , wherein the steel sheet has a coating layer disposed on a surface thereof. 
     
     
       5. A method for producing the steel sheet according to  claim 1 , the method comprising:
 holding a steel slab having the chemical composition for  100  minutes or more with a slab-surface temperature of 1220° C. or more and subsequently hot-rolling the steel slab into a hot-rolled steel sheet; 
 cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet at a cold-rolling ratio of 40% or more; and 
 performing continuous annealing of the cold-rolled steel sheet, the continuous annealing including treating the cold-rolled steel sheet for 240 seconds or more with an annealing temperature higher than 850° C., subsequently reducing the temperature from 680° C. or more to 260° C. or less at an average cooling rate of 70° C/s or more, then performing reheating as needed, and subsequently performing holding at a temperature in a range of 150° C. to 260° C. for in a range of 20 to 1500 seconds. 
 
     
     
       6. The method for producing a steel sheet according to  claim 5 , further comprising performing coating of the steel sheet subsequent to the continuous annealing. 
     
     
       7. A method for producing the steel sheet according to  claim 2 , the method comprising:
 holding a steel slab having the chemical composition for 100 minutes or more with a slab-surface temperature of 1220° C. or more and subsequently hot-rolling the steel slab into a hot-rolled steel sheet; 
 cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet at a cold-rolling ratio of 40% or more; and 
 performing continuous annealing of the cold-rolled steel sheet, the continuous annealing including treating the cold-rolled steel sheet for 240 seconds or more with an annealing temperature higher than 850° C., subsequently reducing the temperature from 680° C. or more to 260° C. or less at an average cooling rate of 70° C/s or more, then performing reheating as needed, and subsequently performing holding at a temperature in a range of 150° C. to 260° C. for in a range of 20 to 1500 seconds. 
 
     
     
       8. The method for producing a steel sheet according to  claim 7 , further comprising performing coating of the steel sheet subsequent to the continuous annealing.